Is This a House or a Playground?

This couple’s 13,000-square-foot home in Sioux Falls, S.D., features a stage with velvet curtains, a hidden room under the basement stairs, a backyard zip line and more—all added to inspire and enrich their children.

Keith and Sara Keane Baumgarten's home in Sioux Falls, S.D., includes an indoor slide, a music stage, and carnival games for their four children. Photo: Dana Damewood for The Wall Street Journal

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Keith and Sara Keane Baumgarten's home in Sioux Falls, S.D., includes an indoor slide, a music stage, and carnival games for their four children. Photo: Dana Damewood for The Wall Street Journal

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Keith and Sara Keane Baumgarten's home in Sioux Falls, S.D., includes an indoor slide, a music stage, and carnival games for their four children. Photo: Dana Damewood for The Wall Street Journal

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Champ, Eileen and Max just got home from school. Champ, 11, heads to the living room, removes his shoes and—swoosh—takes the metal slide to the basement.

On this lower level, their parents, Keith Baumgarten and Sara Keane Baumgarten, have built a vast playland for their four children, ages 4 to 11, in their 13,000-square-foot home in Sioux Falls, S.D. Here, the children practice classical guitar, mixed with some piano and percussion, on a stage flanked by velvet curtains. In a hidden room beneath the stairs, decorated with Chinese lanterns and jade carpet, the children practice Mandarin once a day with a private tutor. Later, they can play skeeball, the classic carnival game, or enjoy a snack from the basement’s kitchen.

In an adjacent guesthouse, there is an elaborate, lofted playhouse with bunk beds for each of the children. Outside, there is a large swimming pool and a play set that includes a climbing wall, firehouse pole and zip line.

The Sioux Falls house is nothing like the couple’s former home. In 2006, Mrs. Baumgarten, a business-management consultant, and her husband, Dr. Baumgarten, an orthopedic specialist, were renting a 611-square-foot apartment on Manhattan’s Upper East Side. With their first child on the way, big-city living was wearing thin.

"I wanted a place to raise the kids where they wouldn’t know the brand names of cars and clothes by grade school," she said. So two months pregnant with their first child, she and her husband took a friend’s advice and moved to Sioux Falls, in southeastern South Dakota.

Dr. Baumgarten, now 44, a former assistant physician with the New York Mets, was offered a partner position at the city’s Orthopedic Institute. Mrs. Baumgarten, 41, left her job shortly after the baby was born.

"We thought we’d give it two years," said Mrs. Baumgarten, who now owns and manages 19 rental properties in Sioux Falls. "Four kids later and we love it here."

In 2009, they bought a house on 2½ acres in a tree-lined neighborhood about a 10-minute drive from downtown, paying $2.05 million, according to county records. With an eye toward making the home fun for their four children, the Baumgartens embarked on a four-year, roughly $1.2 million renovation. They also built a guesthouse with a 2,500-square-foot apartment for Mrs. Baumgarten’s mother, Mary Keane.

"Sara and I made a pact that I would stay and help raise the kids," said Mrs. Keane, 67, who moved to Sioux Falls from Kansas City, Mo., in 2010 after the couple’s third child, Max, was born. Mrs. Keane’s apartment includes the children’s custom bunk beds for weekend sleepovers.

Extracurriculars are a big part of the couple’s parenting philosophy. "I never wanted my kids to say they hate [a subject], because they’re not good at it," Mrs. Baumgarten said. "Also, I like discipline."

To that end, the children take Mandarin and Spanish lessons from two international students who also live at the home. Math and music practice are a daily routine.

Building a fun house is serious work. The 4,000-square-foot basement was jacked up to raise the ceiling height from 8 feet to 12 feet, said Troy Prosa of TAP Architecture, based in Minneapolis.

The family lived for a year with a large hole blown out of the lower level, where tons of dirt had to be excavated.

Mr. Prosa also designed the basement’s musical stage, which has professional lighting and sound. Nearby he created a sleepover room with privacy curtains, lots of plugs for electronics, and a custom-cut Tempur-Pedic mattress on the floor that can sleep eight. The custom-built curly slide gets more use than the stairs.

With four children, a grandmother, two visiting language tutors and two dogs, Spanky and Tigger, the family uses all seven bedrooms and 11 bathrooms. Including the guesthouse and pool house, there are four kitchens and a home office that Mrs. Baumgarten shares with the children during study time.

"We’ll never get our money back for this, but it is so worth it," she said. "I’m never selling."

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